When Ed unloads the rifle in Danny's room, he removes the magazine, but doesn't pull the bolt back to clear the chamber. Ed would surely have know how to properly unload and clear a battle rife.
When Big Ed enters Danny's room, you are presented a view of the strip facing northeast, with the Stardust, and Stratisphere, when Big Ed enters the bedroom, the view faces southwest with the Paris and MGM in view. The shot entering into the hotel window establishes that his room was toward the middle of the hotel, and therefore could only have windows facing the same direction on the strip, not opposite.
Part of the con the 3 bad guys try to claim, is that they went to the wrong Las Vegas, but just what would the odds be that if you mistakenly went to the wrong Las Vegas, and your friend realized this, that he could drive into Vegas and find you especially when none of them had real identification.
Danny checks into a room at Treasure Island. As the camera spins around then focuses on one window, it zooms in on Danny sitting on a chair. The big windows at Treasure Island are actually shared by 4 rooms on 2 floors, not by just a single room. Also Big Ed takes the magazine from Danny's rifle, would have been nice if he would have checked the chamber for a round.
After nabbing one of the Dumb and Dumber brothers, his friends mill around the casino looking for him. Then we see Danny proposing to Mary late in the evening. After a commercial, we have Mary talking to friends over breakfast in the blazing sun, then back to security where the new guy lost the 2 suspects, but backtrack to find then leave the building into sunshine. this implies, the 2 spent well over 8 hours milling around the casino before departing, which in itself would go too far for suspicious behavior.
Ed entered Danny's room and found him sitting there with an M16 by his side. When Danny got up to walk away, Ed picked up the rifle (if it was intended to be Danny's service rifle, it isn't legal for him to own. At a minimum, they can fire three rounds per trigger pull. That's not Ed's mistake though.). Ed wanted to safe the rifle, so he took the magazine out. The problem is, he never cleared the rifle by getting the round out of the chamber. He didn't even check to see if there was one. He would have had to pull the slide back, and the bullet would have been ejected, if there was one. Some one with Ed's experience wouldn't have fogotten that.